The present invention relates to a new glass composition characterized by fast ionic transport comtemplated for use as an electrolyte in electrochemical or fuel cells. The invention further relates to a new method of preparing glasses with improved ionic conduction.
Until recently good ionic conductors were selected almost exclusively from crystalline solid electrolytes. For example, sodium-sulfur electrochemical cells often employ sodium .beta. alumina as an electrolyte for sodium ion transport. Recently, however, there has been work involving glasses that also exhibit ion conduction. Such glasses may offer the advantages of better mechanical properties and isotropic conductivity when formed into thin sheets or disks for use as solid electrolytes.
A great deal of attention has been given to alkali oxides as network modifiers within oxide glasses. For instance both lithium and sodium oxides have been used as modifiers within silicon oxide networks to form amorphous electrolytes. Unfortunately the binary alkali glasses exhibit relatively low electrical conductivities generally in the range of about 10.sup.-4 to 10.sup.-6 (ohm cm).sup.-1 at temperatures of about 300.degree. C. where many of the high specific energy electrochemical cells are operated.
Some improvement in ionic conductivity has been obtained by substituting sulfur for the oxygen within oxide glasses. Sulfide glasses thus prepared are reactive with atmospheric oxygen and thus require special care in preparation and use. In addition, expensive materials such as germanium sulfide have been suggested as the network former in glass systems modified by lithium sulfide and sodium sulfide. Although such glasses present technological advantages in their mechanical properties, their reported conductivities only approach those of the better solid crystalline electrolytes.
Prior methods of preparing a reactive glass that exhibits ionic conductivity involve preparing a powdered mixture of the desired constitutents and sealing the mixture in a vitreous carbon or other high-temperature containment under vacuum. The mixture is melted at 700.degree. to 1000.degree. C. and the glass is formed by immersing the containment in a suitable coolant. Cold water or liquid nitrogen have been suggested as coolants. The glass can be annealed at temperatures typically of about 20-50 degrees lower than the glass transition temperature while sealed in the containment.